Apparatus for visual inspection

ABSTRACT

An apparatus is provided which reduces the dependency of the direction of polarization on channels of an image sensor so as to improve the sensitivity of inspection. In the apparatus, the direction of an illumination beam incident on a polarizing beam splitter is made to be substantially parallel to the longitudinal direction of a field of view of an image sensor projected on the polarizing beam splitter.

RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.11/262,826, filed on Nov. 1, 2005, claiming priority of Japanese PatentApplication No. 2004-320771, filed on Nov. 4, 2004, the entire contentsof each of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to an apparatus for visual inspectionadapted to detect defects of a pattern to be inspected and moreparticularly, to a visual inspection apparatus used for inspectingpattern defects and foreign particles in the course of production of asemiconductor wafer, a photo-mask, a printed circuit board and so on.

In visual inspection of a semiconductor wafer, a method is known asdisclosed in, for example, JP-A-2000-155099, according to which when anillumination beam of rays of light is reflected on the surface of aspecimen and an optical image of the specimen is formed by interferenceof a zero-order diffraction ray and higher-order diffraction rays of areflected beam, the contrast of the optical image is improved to detecta defect of a fine wiring pattern formed on the specimen with highsensitivity. A basic construction of an optical system used for themethod is shown in FIG. 4. A beam of light emitted from a light source 8reaches an aperture stop 11 through a concave mirror and lens 9 and thenis rendered to be incident on a polarizing beam splitter 15 by way of alens, a band-pass filter 12 and a field stop 13. The beam transmitsthrough the polarizing beam splitter 15 and a resultant linearlypolarized light beam leaving the splitter passes through a halfwaveplate 16 and a quarter waveplate 17 so as to be converted into anelliptically polarized light which in turn is irradiated on a specimen 1by means of an objective lens 20. The direction of major axis of theelliptically polarized light can be controlled by rotating the quarterwaveplate 17 and the ellipticity of the elliptically polarized light canbe controlled by rotating the half waveplate 16. A beam of lightreflected from the specimen 1 also enters the polarizing beam splitter15 via the objective lens 20, quarter waveplate 17 and half waveplate 16and then only an S-polarised component is reflected and led to animaging optics comprised of an imaging lens 30 and a zoom lens 50. Theoptical image of the specimen 1 is formed on an image sensor 70. Thenthe image is converted to image data and defects are detected byprocessing the image data.

When, in this imaging optics, angles of the two waveplates are settledsuch that the specimen 1 is illuminated with a circularly polarizedlight, only a component having its polarization state unchanged duringthe reflection on the specimen surface is reflected at the polarizingbeam splitter 15 and led to the imaging optics. On the other hand, whenangles of the two waveplates are settled such that the specimen 1 isilluminated with an elliptically polarized light, part of a componenthaving its polarization state changed during the reflection on thespecimen 1 is also reflected at the polarizing beam splitter 15 and ledto the imaging optics. Generally speaking, a beam of light raysdiffracted by a linear pattern sometimes changes its polarization statebut a zero-order ray does not change its polarization state.Accordingly, by illuminating the elliptically polarized light, thediffracted light component can be emphasized and then led to the imagingoptics.

Hereinafter, the degree of emphasis of a diffracted beam attributable tothe elliptically polarized light illumination will be called an SRintensity. The image sensor 70 is constructed of groups of pixels andtypically, individual groups are called channels. For example, all thepixels on the sensor are divided into 32 or 64 channels.

The SR intensity has dependency on channels of the image sensor.Accordingly, under one illumination condition, the sensitivity differsbetween channels and therefore the condition is settled by a value ofaverage sensitivity. In order to improve the inspection sensitivity, thedifference in sensitivity between channels of the image sensor needs tobe reduced.

An object of the present invention is to realize an optical systemcapable of improving the inspection sensitivity by reducing thedependency of the SR intensity upon channels on the image sensor.

SUMMARY OF THE INVENTION

It has been found out that the direction of incidence of an incidentlight on the polarizing beam splitter has relation to the dependency ofthe SR intensity upon channels. FIGS. 3A and 3B are useful to explainhow the direction of the incident light, the reflection surface ofpolarizing beam splitter and the shape of a field of view of imagesensor projected on the polarizing beam splitter are related to oneanother in the conventional visual inspection apparatus. The directionof an incident illumination light 3100 is substantially vertical to thelongitudinal direction of the field of view of the image sensor, 3300,which is projected on the polarizing beam splitter 15. Because of thesubstantial verticalness of the direction of the incident light to thelongitudinal direction of the field of view of image sensor projected onthe polarizing beam splitter, the incident angle shifts more largely atlongitudinal ends than in the center portion and the direction ofpolarization of the reflected illumination light beam shifts. Since thelongitudinal direction of oblong shape of the image sensor correspondsto the channel direction of the aforementioned 1 to 32 or 1 to 64channels, the dependency on channels is caused as described previously.

In the present invention, an optical system arrangement is adopted inwhich the direction of an incident light beam on the polarizing beamsplitter is so adjusted as to be substantially parallel to thelongitudinal direction of a substantially oblong-shaped view field ofthe image sensor projected on the polarizing beam splitter.

According to the present invention, since the dependency of thedirection of polarization on channels of the image sensor can bereduced, the polarization direction can be optimized easily and areduction in sensitivity depending on channels can be obviated, with theresult that the defect detection sensitivity can be improved as a whole.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an embodiment of a visualinspection apparatus according to the present invention.

FIG. 2A is a top view showing an example of an optical system layoutaccording to the invention in top view form.

FIG. 2B is a side view showing an example of an optical system layoutaccording to the invention.

FIG. 2C is a graph for explaining effect of the optical system layout.

FIG. 3A is a top view showing an optical system layout according to theprior art.

FIG. 3B is a side view showing an optical system layout according to theprior art.

FIG. 3C is a graph for explaining effect of the conventional opticalsystem layout.

FIG. 4 is a diagram showing an example of construction of an opticalsystem of conventional visual inspection apparatus.

FIG. 5 is a schematic diagram showing another embodiment of the visualinspection apparatus according to the invention.

FIG. 6 is a schematic diagram showing still another embodiment of thevisual inspection apparatus according the invention.

FIG. 7 is a diagram showing another example of the optical system layoutaccording to the invention.

DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present invention will now be described withreference to the accompanying drawings.

In an embodiment of an apparatus for visual inspection according to thepresent invention as schematically illustrated in FIG. 1, a specimen 1is chucked on a stage 101 and is movable to optimum positions asnecessary in rotating direction, X direction, Y direction and heightdirection, respectively. A beam of light rays emitted from a lightsource 8 reaches a polarizing beam splitter 15 by way of midway mirror200, sharp-cut filter 300, lens 9, cone lens 700, diffuser 800, NDfilter 900, band-pass filter 12 and opening of aperture stop 11. Thebeam transmits through the filter and a resultant linearly polarizedlight leaving the polarizing beam splitter 15 travels through a halfwaveplate 16 and a quarter waveplate 17 so as to be converted into anelliptically polarized light which in turn is irradiated on the specimen1 by means of an objective lens 20. The direction of major axis of theelliptical polarization can be controlled by rotating the quarterwaveplate 17 and the ellipticity of the elliptical polarization can becontrolled by rotating the half waveplate 16.

A beam of light reflected from the specimen 1 enters the polarizing beamsplitter 15 via the objective lens 20, quarter waveplate 17 and halfwaveplate 16 and only an S-polarized component is reflected, thustraveling through an imaging lens 30 to form an image of the specimen 1on the light receiving surface of an image sensor 2000. Used as theimage sensor 2000 is, for example, a linear sensor or a TDI imagesensor. Part of the light beam reflected from the specimen 1 and thenreflected from the polarizing beam splitter 15 is separated by means ofa beam splitter 1800 and led to an auto-focus system 2100, being usedfor controlling the specimen in the height direction.

A mercury lamp or a xenon lamp may be used as the light source 8 butwith a laser beam source used, a highly intensive photo-output can beobtained in the DUV (Deep Ultra Violet) range.

The image sensor 2000 referred to herein is a TDI image sensor havingits light receiving surface constructed of 256×4096 CCD's and taking arectangular form sized to 3.3 mm×53.2 mm. Further, in the direction of4096 pixels, the 4096 pixels are divided into, for example, 64 channels.Accordingly, in the present embodiment, the TDI image sensor has 64channels in the longitudinal direction of the rectangular form. In FIG.1, the image sensor 2000 is arranged in such a manner that itslongitudinal direction coincides with the right and left direction onthe sheet of drawing and therefore, the image sensor has its field ofview, which is projected on the polarizing beam splitter 15, in thehorizontal direction on the sheet of drawing. Since the direction ofoptical path from the polarizing beam splitter 15 toward the imagingoptics is horizontal on the sheet of drawing in the FIG. 1, thesedirections coincide with each other.

In the visual inspection apparatus of the invention, the polarizing beamsplitter, image sensor view field shape, incident beam and reflectionbeam are related to one another as schematically illustrated in FIGS. 2Aand 2B. In the prior art, like relationship is held as schematicallyillustrated in FIGS. 3A and 3B. FIGS. 2A and 3A are top views and FIGS.2B and 3B are side views. Effects of the present invention will bedescribed by making reference to these Figures.

Firstly, the conventional optical system will be described. In FIG. 3A,an incident light beam 3100 enters the polarizing beam splitter 15horizontally in the left to right direction. As illustrated, the imagesensor view field 3300 projected on the polarizing beam splitter 15 hasits longitudinal direction (channel direction) coincident with thevertical direction. A reflection surface 3400 of the polarizing beamsplitter is related to a reflection beam 3200 as shown in FIG. 3B. FIG.3A shows a direction of polarization in the central part of the imagesensor view field, 3600 b, and directions of polarization at the ends ofthe image sensor view field, 3600 a and 3600 c. Incident light raysreaching the individual positions (individual channels) of the imagesensor view field have been incident on the reflection surface 3400 atdifferent angles, so that the polarization directions 3600 a, 3600 b and3600 c differ slightly from one another. As the result, the dependencyof SR intensity on channels of the image sensor occurs as shown in FIG.3C.

Next, an optical system of the present invention will be described. Inan layout of the invention as shown in FIG. 2A, the longitudinaldirection (channel direction) of an image sensor view field 3300projected on the polarizing beam splitter 15 coincides with thedirection of an incident illumination light beam 3100. With thisconstruction, incident light rays reaching the individual positions(individual channels) of the image sensor view field have been incidenton a reflection surface 3400 at the same incident angle, so that thepolarization directions 3500 b, 3500 a and 3500 c in the center and atthe opposite ends of the image sensor view field, respectively, areidentical with one another. As the result, the dependency of SRintensity on channels of the image sensor does not occur as shown inFIG. 2C.

Another example of the optical system according to the invention isillustrated in FIG. 7. In this example, an illumination light beam fromthe light source enters the polarizing beam splitter from above and abeam transmitting through it illuminates the specimen. A reflected beamfrom the specimen is reflected at the reflection surface of thepolarizing beam splitter and led to the imaging optics. In this layout,the longitudinal direction (channel direction) of image sensor viewfield projected on the polarizing beam splitter is substantiallyparallel to an optical path extending from the polarizing beam splitterand ending in the imaging optics. With this construction, anillumination light ray illuminating the center of the image sensor viewfield and illumination light rays illuminating the opposite ends thereofhave the same incident surface in relation to the reflection surface ofthe polarizing beam splitter, thereby ensuring that polarizationdirections of these illumination light rays on the image sensor viewfield can all be parallel to the longitudinal direction of the imagesensor view field. As a result, the illumination characteristics have nodependency upon channels of the image sensor and the SR intensity doesnot sustain the dependency on channels.

Another embodiment of the visual inspection apparatus according to theinvention is schematically illustrated in FIG. 5. The apparatus shown inFIG. 5 differs from the visual inspection apparatus shown in FIG. 1having the half waveplate 16 and quarter waveplate 17 arranged betweenthe polarizing beam splitter 15 and objective lens 20 in that only aquarter waveplate 17 is interposed. The visual inspection apparatusconstructed as shown in FIG. 1 is suitable for use with illumination byan elliptically polarized beam but the apparatus constructed as shown inFIG. 5 is effective when a circularly polarized beam is used forillumination.

Still another embodiment of the visual inspection apparatus according tothe invention is schematically illustrated in FIG. 6. The apparatusshown in FIG. 6 is also identical to the visual inspection apparatushaving the half waveplate 16 and quarter waveplate 17 arranged betweenthe polarizing beam splitter 15 and objective lens 20, with the onlyexception that a half waveplate 16 alone is interposed. This visualinspection apparatus is effective when a linearly polarized beam is usedfor illumination.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A visual inspection apparatus comprising: a specimen stage forholding a specimen; a light source; a polarizing beam splitter; anobjective lens; a quarter waveplate arranged between said polarizingbeam splitter and objective lens; an imaging optics; and an imagesensor, an illumination beam of light rays emitted from said lightsource being irradiated on the specimen held by said specimen stage viasaid polarizing beam splitter, quarter waveplate and objective lens anda reflection beam from said specimen being led to said imaging optics byway of said objective lens, quarter waveplate and polarizing beamsplitter so as to be imaged on said image sensor, wherein theillumination beam from said light source is rendered to be incident onsaid polarizing beam splitter in an incident direction which issubstantially parallel to a channel direction of a field of view of saidimage sensor projected on said polarizing beam splitter, or thedirection of an optical path extending from said polarizing beamsplitter toward said imaging optics and ending in said imaging opticalsystem is substantially parallel to said channel direction. 2-6.(canceled)